Threaded pipe connections that connect joints of pipe together and seal between them to prevent leakage of fluids from the pipe, have been used for hundreds of years with the external thread lead being made as close as then possible to the internal thread lead. Most pipe connections threads in use today are tapered threads, so the interface pressure between the threads will increase with each turn of makeup to a degree of tightness deemed sufficient to prevent loosening of the connection and leakage of fluid from the pipe. If interface pressure is too low the threads will loosen and leak but if it is too high, the pin or the box may be yielded plastically such that its operability is questionable. As pipe sizes and fluid pressures increased, pipe materials graduated from bamboo, to wood, to brass, to iron, to steel and occasionally to high-strength, high-temperature alloys as necessary to maintain strength and sealability which in turn, required an increase of radial thread interference and interface pressures between the mating threads. For purposes of this application: A thread flank angle is measured in a plane coincident with the pipe axis, in the gap between the flank surface and a plane perpendicular to the pipe axis; A pin is a tubular member having external threads; A box is a tubular member having mating internal threads for assembly with the pin; One thread turn means one revolution of a thread; Lead means the axial length a thread advances in one thread turn; A negative flank means a flank that faces more toward the root; A positive flank means a flank that faces more toward the crest; A hook-type thread is a thread with a negative flank angle; “My invention” is the subject of the present application.
Historically, pin threads have been made with the same taper and lead as the mating box threads until Watts U.S. Pat. No. 4,974,882 taught that the pin thread should be made with a slower taper than the box thread so that upon assembly, the maximum radial interference between mating threads occurs at the small diameter end of thread engagement which facilitates assembly of the pin with the box, improves thread strength and assists sealability.
Sivley U.S. Pat. No. 6,976,711 teaches changing of the stab flank lead and/or the load flank lead of mating box and pin threads such that the “stab flank lead and the load flank lead become equal to the average lead at a selected distance from the end of the threads” and “the load flank lead and the stab flank lead are different from each other at least part of the thread length” but nowhere does '711 teach changing the pin thread lead relative to the box thread lead, nor does it mention any thread lead mismatch that is created during assembly of pin with box threads due to Poison's Ratio. In view of Invitroge v. Clontech Lab nos. 04-1039-1040 patent, '711 is not prior art because it neither mentioned the problem nor the solutions taught herein. To applicant's best knowledge and belief, there have been no compensations taught to insure intimate contact between mating threads or to offset mating thread lead mismatch caused by radial interference.
To illustrate some of the problems solved by my invention, Type 1 Failures depicted in FIG. 1 often occur in prior art such as API 5B 8-Round pipe threads when assembly of box (57) with pin (50) compresses the pin radially causing it to elongate axially, and also expands the box radially causing it to shorten axially, the difference in lengths forcing pin stab flank (52) against thirty degree box stab flank (53) whereupon the radial force vector of the axial force moves pin face (54) inwardly and out of engagement with mating box thread (55) which weakens the connection because fewer threads are then in contact to resist axial loads, and it also opens helical leak path (56) between the mating threads.
FIG. 2 depicts a Type 2 failure of conventional threads such as API 5B 8rd pipe threads after being subjected to external fluid pressure acting around box (61), external fluid pressure increasing the compressive hoop stress in pin face (62) due to thread interference upon assembly. The combination of forces plastically yield the pin-end, causing loss of contact between box thread (63) and mating pin thread (64) which upon release of the external pressure, leaves open both axial and helical leak path (67) which prevents the threads ability to mate and seal against internal fluid pressure after the still elastic box has returned to its original diameter and the yielded end at the pin face has not because the compressive hoop stress is greatest at the pin face. Type 1 and Type 2 failures tend to occur in connections where pin thickness near the pin face is less than adjacent box wall thickness.
FIG. 6 depicts first contact between conventional threads such as API 5B 8Rd threads when stabbed at the rotational position that effects the least pressure angle (68) for that thread form, the pressure angle being formed between pipe axis (25) and force vector (46) that passes through mutual contact point (93), the least possible pressure angle being equal to stab flank angle (81). Such threads have pin stab flank (70), crest (74), box stab flank (71) formed on stab flank angle (81). FIG. 7 depicts first contact between pin and box threads when stabbed at the rotational position that effects very high pressure angle (79) which may cause a Type 3 Failure, as pin crest (74) contacts box stab crest (77), the failure being caused by too great a pressure angle (79) which generates too great a bearing stress between the mating stab flanks at mutual tangent point (82) in the direction depicted by force vector (76). Pin stab surfaces include surface (74) formed intermediate pin stab flank (70) and pin thread crest (74). Box stab surfaces include surface (75) formed intermediate box stab flank (71) and box thread crest (77). Surfaces (74, 75) may be formed arcuate, conically or otherwise as is best to prevent handling damage to sharp corners that would otherwise be positioned there, as is well known in the art. API Buttress and other thread forms having cylindrical crests with radii between the flank and crest are affected likewise. After a pin is stabbed into a box, it is typically rotated a partial turn opposite the direction of makeup in an attempt to attain the rotational position of minimum pressure angle for that thread design as depicted in FIG. 6. Depending on the rotational position between the box and pin thread when stabbed, the pressure angle may vary from, a minimum equal to flank angle (81) to almost 90 degrees as depicted in FIG. 7 at (79). An increase of the pressure angle increases the unit bearing stress between the threads as at mutual tangent point (82), which increases torque required to rotate the pin into the box against the friction force caused by the weight of pipe that is being stabbed. As the pressure angle increases the vector force increases, and depending on the weight of pipe being stabbed, the friction force can become so great that the threads gall, cross-thread and/or lock-up, which prevents proper assembly of the pin with the box. The smaller the crest radius and the faster the reverse rotation, the more likely that pipe inertia during reverse rotation will cause the pin to pass the rotational point of failure before gravity lowers the pin one thread turn to gain best position, however, when past the best position depicted in FIG. 6, the danger of a Type 3 failure is again imminent within less than a turn.
Swaged box connections used over the years have typically been rated at 65% of the pipe strength and a few have nervously been rated toward 75% of the pipe body strength when formed on certain thick-wall pipes. They could claim no higher ratings because thread engagement did not extend completely through the box wall, but stopped in a swaged portion formed on the same taper as the threads, which reduced the critical area at the neck of the box to no more than 75% of the pipe wall cross-sectional area if the threads were perfectly positioned both axially and radially with regard to the swage wall. However, such perfect positioning cannot be maintained during production threading, so the resulting efficiencies of prior art swaged connections have been typically limited to less than 75% of pipe strength.
API Reports state that ninety to ninety-five percent of down-hole well problems are caused by leaking pipe threads and API records are replete with extensive experiments that attempted to learn why pipe threads leak in service. In error, API long assumed that all API 5B pipe threads coated with API 5A2 thread dope sealed, as confirmed by the fact that 5B thread dimensions and tolerances and API 5A2 thread dopes were not substantially changed since they were adopted in 1939, until after the 1995 issue of Watts' U.S. Pat. No. 5,427,418 that taught solution. Currently, no prior art has been found by applicant that teaches: Upon assembly, mating threads formed with equal axial lead lengths are subject to Types 1, 2 and 3 Failures, and no solutions have been taught to solve the problems, so the problem identifications and solutions in this application are presented to advance the art.